The well-known broadband characteristics of optical fibers make such fibers a prime candidate for intercontinental and other undersea communications. Additionally, for such intercontinental and other undersea applications, which most often cover significant lengths, it is important to employ a transmission system that requires relatively few repeaters per given distance. However, such applications of course require that the fiber core unit be incorporated into a cable design that provides adequate protection from the various environmental elements likely to be experienced by a cable used in such intercontinental and other undersea applications.
One major problem inhibiting the use of optical fibers in a submarine cable is the necessity of hermetic protection of the fibers against moisture. This is particularly significant because of the mechanical stresses on a submarine cable during laying and recovery, since the combination of moisture and stress on an optical fiber has been found to quickly lead to structural failure. Moreover, a fiber-bearing cable must provide an efficient DC path for powering the optical repeaters in the system and must be sufficiently strong to withstand the above-mentioned stresses and years of operation at sea.
The reconciliation of all of these requirements in a single submarine cable has up to now been somewhat less than desirable with regard to at least some features. For example, while certain manufacturers presently offer multi-element optical fiber arrays which are arranged in cable form (including strength members and packing), such designs cannot reliably withstand the pressures and stresses of operation at sea, nor are they sufficiently waterproof to prevent deleterious moisture penetration to the fibers. Specific examples of existing cable designs for undersea applications are set forth in commonly assigned U.S. Pat. Nos. 5,222,177 and 5,224,190 issued in the name of Chu et al., on Jun. 22, 1993 and Jun. 29, 1993 respectively.
However, another specific cable design presently used by the assignee of the present application is disclosed in commonly-assigned U.S. Pat. No. 4,156,104 issued on May 22, 1979 in the name of Mondello, and is incorporated by reference herein. In particular, the immediately aforementioned patent discloses a repeatered submarine cable having a composite inner conductor for accommodating a system of optical fibers therein. The cable strength members include a central elongated filament and plural layers of stranded steel wires separated from the central filament by an annular insulating core member, in which the optical fibers are embedded. More specifically, the annular insulating member set forth is a polyether polyester elastomer supplied under the designation HYTREL.TM. by the Dupont Corporation. Additionally, a metallic tubular jacket surrounds the stranded steel layers to provide both a DC path for powering the optical repeaters and a hermetic moisture barrier for the fibers.
While the cable design described above offers advancement over previously available designs, there remains a continuous effort to improve additional characteristics of the various sections of the cable in order to enhance the overall operation of the communications cable. One particular area of aggravation associated with the use of HYTREL.TM. as the annular insulation material is that when fiber splicing becomes necessary, an operator in the field must use a chemical solution to remove the HYTREL.TM. to gain appropriate access to the fibers. At present, the particular chemical most often used by industry is methylene chloride, CH.sub.2 Cl.sub.2. The use of this chemical is not only messy and cumbersome but, also may damage some solvent-based color codings that may be applied to the fiber. Therefore, many of the color-coding techniques presently used throughout the industry are left ineffective after treatment with common chemicals, particularly CH.sub.2 Cl.sub.2. In addition, the use of methylene chloride, CH.sub.2 Cl.sub.2 introduces certain health and/or safety concerns that must be addressed in order to appropriately protect an operator from harm when directly exposed to such material. In summary, requiring an operator to use such a treatment on the fiber to properly prepare it and to allow adequate access to the individual fibers for fiber splicing, establishes a substantial burden and inconvenience for the operator and also greatly interferes with the operational effectiveness of the overall communication cable.
The present invention overcomes the above-stated problems with existing cable designs by utilizing a elastomeric material as an annular insulating layer. Notably, the preferred elastomeric material used in accordance with the present invention provides significant improvements over the materials used in cables now available. More specifically, the sought-after cable structure includes a material around the fiber which exhibits sufficient inter fiber mobility to allow movement of the fibers during handling and installation without damaging the fiber structure. Also, the sought-after structure should be mechanically rugged to withstand cabling operations and plowing of the cable into the ground during installation and should exhibit acceptable loss performance at temperatures as low as -40.degree. F. Notwithstanding these requirements, the fiber configuration should be compact in size, and be strippable with access to the individual optical fibers from any end of the fiber configuration, or from midspan, without removing any coloring material from the fibers and without the need for complex tools.